Rust: Restrict type propagation into arguments

[hvitved]

Type propagation into arguments is needed in cases like

let x = Vec::new();
x.push(0); // the type `i32` must propagate into `x` to infer that `x` has type `Vec<i32>`

Up until now, we have unconditionally propagated types into all arguments, which can sometimes lead to explosions in inferred types.

This PR circumvents such explosions by only propagating type information into arguments when those arguments are potentially context-typed (such as Vec::new and Default::default).